Processing of Luran S 777K

General information

Luran® S can be processed by any of the processes suitable for thermoplastics. The most important processes are injection molding and extrusion.

Predrying

Luran® S granules can absorb small amounts of moisture from the air during storage. Although this does not change the properties of the product, streaks or bubbles can appear during processing, depending on the moisture content. We therefore recommend predrying of Luran® S before processing. The Luran® S grades which have a "C" in their designation (eg. Luran® S KR 2861/1 C) are blends of ASA and polycarbonate. These products should always be predried. The drying temperature should be about 10-20°C lower than the Vicat softening point (VST/B/50). The drying conditions are given in the following Table.

Because of its high thermal stability, up to 30% of clean Luran S of a single grade and whose previous processing has not involved contamination may be added to the starting material in the manufacture of moldings where this is appropriate. The usual crushers or granulators can be used for size reduction. Regrind should be well dried.

Injection molding

Luran S molding compounds can be processed on any commercially available injection molding machine. Single-screw injection molding machines are usually used.

Screw configuration

A conventional 3-zone general purpose screw fitted with a non-return valve can be used. The screw length should be from 16 to 20 D. The data given in the following Tables are typical values for screw designs which have proven successful. The pitch is constant over the entire length and should be from 0.8 to 1 D.

Typical values for screw configuration:

Length of section

Overall length

16 -20 D

Feed zone

8 - 10 D

Compression zone

4.8 - 6 D

Metering zone

3.2 - 4 D

Screw diameter (mm)

Flight depth in the feed zone (mm)

Flight depths in themetering zone (mm)

30

5

2.5

50

6

3

70

8

4

Nozzles

Open nozzles can be used for processing Luran S molding compounds because Luran S melts are relatively viscous. Open nozzles have a very simple design and therefore give particularly good flow.Shut-off nozzles have advantages when high back pressure is being used or undesirable stringing has to be avoided and thick-walled parts are to be manufactured. Mechanically or hydraulically operated needle valve nozzles have proven the most successful.

Gate and mold design

Any known type of gate may be used, including hot runner systems. The guidelines for the design of gates and molds for the manufacture of injection-molded parts from thermoplastics (VDI 2006) are also applicable to Luran S. Gates and feed channels should not be too small, otherwise excessively high melt temperature and injection pressure are required. This can result in streaks, charring caused by shear, voids or sink marks.

Use of inserts

Metal parts can be molded in without difficulty, but they should be preheated to 80-120°C before being placed in the mold so that no internal stresses are created. The metal parts must be free of grease, and to improve anchoring should have milled, grooved or similar surfaces. Metal edges should be well rounded.

Mold temperature control

A carefully designed temperature control system for the mold is particularly important, since the effective mold surface temperature has a decisive effect on surface quality (gloss, flow lines) and on the weld line strength, distortion, shrinkage and tolerances of moldings. The recommended mold surface temperatures for Luran S grades are given in the following Table. An eventual warpage of the moldings can be counteracted by separate and differentiated temperature control of the two halves of the mold.

Recommended mold surface temperatures for Luran® S:

Processing temperature (°C)

Mold temperature (°C)

Typical shrinkage (%)

Luran S - ASA grades

240-280 °C

40-80 °C

0.4-0.7

Luran S - ASA/PC grades

260-300 °C

60-90 °C

0.3-0.7

Luran S KR 2867 C WU

260-280 °C

40-60 °C

0.3-0.7

Processing temperature

Luran S molding compounds are generally processed at melt temperatures of from 240 to 280°C, but the polycarbonate-containing Luran® S grades (eg. Luran® S KR 2861/1 C) should be processed at 260 to 300°C except the flame-retardant grade Luran® S KR 2867 CWU, for which a temperature range of 260 to 280°C is recommended (see Table 4 above). For processing at the upper end of the temperature range short residence times should be used, since otherwise the material can undergo thermal degradation. This can be recognized in colored compounds by the change in color; it normally becomes somewhat paler.

Feed characteristics

Even at high screw rotation rates plastification of Luran S molding compounds proceeds smoothly and without thermal degradation. Plastification performance rises with increasing processing temperature.For high processing temperatures and/or for long cycle times the temperature of the first heater band (close to the feed hopper) should be set somewhat lower in order to prevent premature melting of the granules in the feed zone (bridging).

Mold filling

A relatively high injection speed is useful since little cooling occurs during mold filling; this gives a glossy surface, a low visibility of the weld lines and a high weld line strength. Too low a rate of mold filling gives parts with unsatisfactory surfaces. When the melt is injected, care should be taken that the air in the mold cavity can escape at a suitable point, to avoid charring by compressed air (diesel effect). To obtain perfect injection-molded parts and to prevent the formation of voids, the hold pressure and the hold pressure time must be sufficient to compensate the volume reduction which occurs when the melt is cooled. On the other hand overfeeding of the mold cavity must also be avoided, since this causes stresses in the molding. The risk of overfeeding exists mainly in the vicinity of the gate, at high injection rates and high hold pressure.

Luran S can be readily demolded, so that even moldings of complicated design are possible. Drafts of from 0.5 to 0.9° are generally sufficient. Textured surfaces require larger drafts: 1° makes it possible to demold a part with 0.02 mm depth of texture from the mold cavity and a part with 0.01 mm from the mold core.

Shrinkage and post-shrinkage

Shrinkage is significantly lower with Luran S molding compounds than with semi crystalline plastics.The processing shrinkage is usually from 0.4 to 0.7%, and in exceptional cases well below 0.4%. In regions of a molding which experience high hold pressure (near to the gate) it may even be close to 0%.Post-shrinkage is negligible in most applications, making up about 1/10 of the overall shrinkage.

Extrusion

Luran S is highly suitable for the extrusion of sheets, solid and hollow profiles and pipes. Specific extrusion grades are available for this and are identified with an "E" (e.g. Luran S 797 SE).

Sheet manufacture

Suitable extruders for the manufacture of extruded sheets comprise those generally used for processing impact-modified polystyrene and ABS, having a sheet extrusion die. Vented extruders with a screw length of from 25 to 30 D and a compression ratio of from 1:2 to 1:4 are preferable.In most cases the optimum throughput of sheets with superior mechanical properties and good appearance is achieved at about 230°C for the ASA grades or at about 260°C for the ASA/PC grades.

Pipe and profile manufacture

The same conditions apply for the manufacture of pipes and profiles as for the extrusion of sheets. The melt temperature, however, is generally set lower in order to achieve sufficient melt strength between die and calibrator. The recommended lower limit here is 200°C. Preferably, the cooling conditions should be set to give an external temperature of about 70 to 80°C for the semi-finished products after passing through the water bath.

Blow molding

The Luran S grades most suitable for blow molding are those with low flowability. It should be extruded at a melt temperature of from 220 to 230°C. In individual cases, the melt temperature can be lowered to about 210°C in order to reduce stretching of the extruded tube. Short ejection cycles, as achieved when melt accumulators are used are advantageous. Undercuts in blowing molds, e.g. at the thread runout or as a result of the curvature of bottle bases, should be avoided. The pinch-off areas can be designed in the same way as for PE-HD processing. The pinch-off edges should be as sharp as possible to ease flash removal. In the case of engineering parts, the flash areas should be sectioned off by well-defined pinch-offs.

Thermoforming

Sheets and films made from Luran S can be thermoformed to give moldings with good wall thickness distribution. Thermoforming of Luran S can be carried out with standard machinery for the vacuum forming and compressed-air forming of sheets and films. Recommended forming temperatures are from 140 to 170°C.

Storage and packing of thermoforming sheet

Like ABS, Luran S tends to absorb moisture under poor conditions of storage. On thermoforming, sheets which have become moist can generate bubbles which make the molding unusable. Storage in dry areas (about 20°C, 30% relative humidity) prevents the absorption of moisture which impairs thermoforming. However, if storage takes place under standard conditions of temperature and pressure (DIN 50014-23/50-2), the moisture content may reach levels which could adversely affect processing after some days or weeks.

Post-processing

Machining

Semi-finished products made from Luran® S are easy to machine, i.e. die-punch, saw, drill, mill, turn etc., using conventional metal- and woodworking machinery. Tools used for machining brass and bronze are suitable. Because heat dissipation is slow, water cooling is frequently necessary even at low cutting speeds. Luran S parts can be stamped and flanged without difficulty and can be fastened using self-tapping screws.

Welding

Hot-plate and spin welding are suitable for welding semi-finished products and moldings made from Luran S, and in specific cases high-frequency and ultrasonic welding can also be used.Ultrasonic welding can also be used to connect Luran S to a number of other thermoplastics, such as SAN, ABS, PVC and PMMA.

Adhesive bonding

Solvents auch as methyl ethyl ketone, dichloroethylene and cyclohexanone dissolve Luran S and can therefore be used to join Luran® S parts with parts made from Luran S, SAN or ABS. In any case prior trials in order to determine the suitability are necessary. For more information we recommend contacting the adhesives industry, which offers a wide variety of suitable special adhesives.

Surface treatment

Parts made from Luran S may be easily and permanently printed, coated or painted without any special pretreatment. They can also be metallized by the metallizing processes commonly used in industry.

Safety precautions during processing

No adverse effects on the health of processing personnel have been observed when processing of the products is carried out correctly and there is suitable ventilation of the production areas. The maximum allowable concentrations (MAC) of

Acrylonitrile is a Group III, A2 substance for which carcinogenic activity is assumed as a result of toxicity testing. Experience has shown that when Luran S is processed correctly with appropriate ventilation, the levels are far below the limits mentioned above. Inhalation of the vapours of degradation products which can arise on severe overheating of the materials or during pumping out should be avoided. Please consult our material safety data sheets.